minor symfony#11424 fix (serializer): invert xml and yaml bloc descriptions (GregoireHebert)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes symfony#11424).

Discussion
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fix (serializer): invert xml and yaml bloc descriptions

Commits
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508a186 fix (serializer): invert xml and yaml bloc descriptions

Author: javiereguiluz

components/serializer.rst

@@ -258,14 +258,14 @@ for each format:
 
     $classMetadataFactory = new ClassMetadataFactory(new AnnotationLoader(new AnnotationReader()));
 
-* XML files::
+* YAML files::
 
     use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
     use Symfony\Component\Serializer\Mapping\Loader\YamlFileLoader;
 
     $classMetadataFactory = new ClassMetadataFactory(new YamlFileLoader('/path/to/your/definition.yml'));
 
-* YAML files::
+* XML files::
 
     use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
     use Symfony\Component\Serializer\Mapping\Loader\XmlFileLoader;